1. Field of the Invention
The invention relates to a recording apparatus such as a facsimile apparatus or the like and, more particularly, to a fan control of a recording apparatus having a heat fixing heater and a fan.
2. Related Background Art
Hitherto, a halogen lamp is used as a heater of a heat fixing device which is used in a recording unit or the like of a laser beam printer facsimile apparatus (hereinafter, referred to as an LBPFAX). The heater by the halogen lamp has characteristics such that a temperature of the fixing unit of the heat fixing device is gradually raised because a heat capacity of the heater itself is large. In this case, the temperature rising characteristics of the fixing unit are gentle.
When considering the case where a power source of the apparatus is turned on in a cooled state of the fixing unit, it takes a long time until the fixing unit can execute a fixing operation. Therefore, in the ordinary state, after the power source was turned on, the temperature is adjusted so as to keep the temperature of the fixing unit constant.
The temperature at the time of such a temperature adjustment is set to a very high temperature such as 180.degree. C. or the like. When the temperature adjustment is executed, therefore, the temperature and moisture in the apparatus cause problems. In order to reduce the temperature and moisture in the apparatus, a fan is ordinarily attached to the apparatus. After the power source was turned on, by continuously rotating the fan, the temperature in the apparatus is reduced.
The control of the fan, on the other hand, is designed on the basis of the idea of the fail safe. Although a mode to change a rotational speed of the fan on the basis of an instruction from a CPU exists in the control circuit, a mode such as to stop the rotation of the fan does not exist on an electric circuit.
Therefore, even when an image is not printed in a silent office or the like, the fan is always rotated, so that the rotating sound is very noisy. There is also a drawback such that since the fan is always rotated, an electric power consumption is very large.
To prevent such drawbacks, in recent years, a heat fixing device in which temperature leading characteristics of the fixing unit were made sharp has been developed.
The heat fixing device is constructed by a pressure roller, a film, and a heater.
The film included in the heat fixing device is a cylindrical film made by polyimide or the like and has a thickness of about tens of microns. The film is come into contact with the pressure roller included in the heat fixing device through a paper in a paper passing state and is directly come into contact with the pressure roller without interposing the paper in a paper non-passing state.
On the other hand, the heater included in the heat fixing device is formed in an elongated plate shape or a rod shape. A resistance material is coated onto the surface of the heater. The heater exists in the circle of the cylindrical film and is come into contact with the film and is, consequently, come into contact with the paper or pressure roller through the film (hereinafter, such a heat fixing device is referred to as a surf fixing device).
In case of the surf fixing device, it is sufficient to heat the heater just before printing after the power source was turned on. Therefore, generally, no electric power is applied to the heater, in other words, the temperature adjustment of the heater is not performed in a waiting state. Therefore, it is proper to construct the apparatus so as to rotate the fan in accordance with the necessity when the electric power is applied to the heater, namely, when the temperature of the heater is adjusted.
In the above surf fixing device, however, the film made by polyimide or the like generally has a thickness of about tens of microns and is very thin. Therefore, when an excessive temperature increase occurs on the surface of the film, namely, in the contact portion with the heater or the contact portion with the pressure roller or recording paper, there is a serious problem such as occurrence of damage, smoke emission, or fire production of the film. Consequently, there is an anxiety.
On the other hand, it is technically impossible to detect the surface temperature of the contact portion of the film by allowing a thermistor which is generally used to come into contact with the contact portion because of the problems on durability and reliability of the thermistor, film, and pressure roller.
Therefore, since there is a danger of the occurence of damage, smoke emission, or fire production of the film, for example, a vain operation for rotating the fan is executed in spite of the fact that the temperature of the film is low.
In a recording apparatus such that the temperature is detected and the fan is rotated by a control unit such as a CPU or the like, there is a possibility such that the CPU or the like runs away and the apparatus is subjected to a very dangerous state due to an increase in temperature.